1. Field of the Invention
The present invention relates to a non-directional condenser microphone in which a rear space behind a diaphragm is substantially sealed, and to a condenser microphone unit and a condenser microphone provided with a pressure equalizing communication passage which prevents the diaphragm from being displaced with changes in atmospheric pressure, for example.
2. Description of the Related Art
A non-directional condenser microphone is fundamentally such that a rear space behind a diaphragm is sealed and the diaphragm is displaced according to a difference between sound pressure applied to a sound terminal outside (in front of) the diaphragm and pressure in the above-mentioned rear space.
This arrangement provides the non-directional microphone which responses only to loudness of sounds regardless of the direction and angle of the diaphragm in the microphone unit.
FIGS. 8 and 9 show an example of the above-mentioned the non-directional condenser microphone unit. FIG. 8 is a sectional view showing a situation where the microphone unit is assembled. FIG. 9 is an exploded sectional view showing the above-mentioned unit, separated into the principal parts.
The condenser microphone unit has a capacitor element in which the diaphragm vibrated by a sound wave and a fixed electrode (back plate) are opposed to each other through an air layer with a predetermined interval, and this capacitor element is assembled in a unit case 1.
That is, the above-mentioned unit case 1 has many sound introduction holes 2 on the front side and is arranged to be in the shape of a cylinder whose rear side is open. This unit case 1 is made of metal materials, such as for example, brass. Into this unit case 1, from its rear side, a front mesh 3, a ring-shaped diaphragm holder 4, a similarly ring-shaped spacer 5, a fixed electrode 6 formed of metal materials, such as brass, and an insulation seat 7 molded from a synthetic resin, etc. are inserted in this order.
Further, a diaphragm 8 vibrated by sound pressure which is applied to a sound terminal is attached to a surface, of the above-mentioned diaphragm holder 4, facing the above-mentioned fixed electrode 6. This diaphragm 8 is arranged to face the above-mentioned fixed electrode 6 through the air layer corresponding to a thickness of the above-mentioned spacer 5 made of a synthetic resin sheet formed in the shape of a ring.
The above-mentioned fixed electrode 6 is supported by the insulation seat 7 so that it may be electrically insulated from the unit case 1 and the diaphragm 8. Further, a pick-up electrode rod 9 for picking up a signal from the above-mentioned fixed electrode 6 is attached to the center of the insulation seat 7.
It should be noted that a cover 10 is attached to the back of the above-mentioned insulation seat 7 in an air-tight manner; air rooms 11 are formed respectively between the insulation seat 7 and the fixed electrode 6 and between the insulation seat 7 and the cover 10, and interconnected through a communication hole 7a bored in a proper position of the above-mentioned insulation seat 7.
These air rooms 11 are connected with the rear space behind the diaphragm 8 through communication holes (sound holes: not shown in FIG. 8 or 9) formed in the above-mentioned fixed electrode 6.
Further, a lock ring 12 is screwed into the rear of the unit case 1 using a female screw formed in the inner periphery of the unit case 1. This lock ring 12 applies predetermined pressure to the fixed electrode 6 through the insulation seat 7 towards the diaphragm holder 4. All the unit components including the diaphragm holder 4 and the fixed electrode 6 are fixed in the unit case 1.
It should be noted that, as with the unit case 1, the above-mentioned lock ring 12 is formed of metal materials, such as brass, for example.
According to the above-mentioned microphone unit (shown by the same reference numeral 1 as that for the unit case), the diaphragm 8 is held by the above-mentioned diaphragm holder 4 at the front of the above-mentioned unit case 1 in the air-tight manner. Thus, as the atmospheric pressure applied to the sound terminal at the front of the diaphragm 8 changes, the diaphragm 8 is displaced according to an atmospheric pressure difference between a space in front of the diaphragm 8 and the rear space including the above-mentioned air room 11. It follows that output sensitivity of the microphone unit 1 changes with the displacement of this diaphragm 8.
In order to prevent the diaphragm displacement caused by such changes in atmospheric pressure, a structure of the condenser microphone provided with a communication passage referred to as a capillary vent (Capillary vent) which allows the rear space (including the above-mentioned air room 11) of the diaphragm to communicate with the outside at a frequency band which is much lower than a sound-collecting frequency band is disclosed by John Eargle, The Microphone Book: (Focal Press), p 49, Figures. 3-20.
Preventing the displacement of the diaphragm caused by change in atmospheric pressure as described above is referred to as “pressure equalization”. As to the pressure equalization, it is necessary for the communication to be carried out at a frequency much lower than the sound collecting frequency band, and it is necessary for the air room to communicate with the open air at a higher acoustic impedance than an acoustic impedance of the air room.
In order to stably obtain high acoustic resistance, a thin pipe (capillary tube) or a thin air layer resistor surrounded by plates is used. Each of these needs a micro fabrication in order to obtain a high impedance, and high cost is unavoidable in order to maintain suitable processing accuracy.
Incidentally, in this type of condenser microphone unit, a structure is employed in which a ring-shaped spacer made of a synthetic resin is interposed between the diaphragm and the perimeter of the fixed electrode so that a diaphragm assembly is attached. FIGS. 10 to 12 illustrate an example of the structure.
It should be noted that, in FIGS. 10 to 12, parts which function similarly to those illustrated in FIGS. 8 and 9 above are denoted by the same reference signs. Accordingly, the description of these parts will not be repeated herein.
FIG. 10 is a sectional view showing a situation where the microphone unit is assembled similarly to the FIG. 8 situation, FIG. 11 is an enlarged sectional view showing a portion indicated by reference sign b in FIG. 10, and FIG. 12 is a front view showing an arrangement of the spacer used in the microphone unit as shown in FIG. 10.
In the microphone unit shown in FIGS. 10 to 12, the spacer 5 with the arrangement shown in FIG. 12 is used. That is, a part of the ring is excised, and the spacer 5 shown in FIG. 12 is arranged such that this excised part 5a may function as an atmospheric gas communication passage (acoustic resistance).
Its feature is expanded and shown in FIG. 12. The expanded sectional view shown in FIG. 11 illustrates the portion including the excised part 5a in the above-mentioned spacer 5.
In addition, in this example, as shown in FIG. 11, a mesh-like spacer (stainless steel mesh) 14 which is obtained by using a stainless steel material (for example) and processing it into the shape of a mesh is arranged at the front side of the diaphragm holder 4.
Being processed in the shape of a mesh, this mesh-like spacer 14 has an air permeability and is formed in the shape of a ring as described above.
According to the above-mentioned arrangement, the communication passage (acoustic resistance) of the excised part 5a cut off at the above-mentioned spacer 5 is formed at a part of a place where the circumferential edge of the above-mentioned diaphragm 8 and the above-mentioned fixed electrode 6 face each other.
Thus, as shown by a dotted line arrow in FIG. 11, the rear space between the diaphragm 8 and the fixed electrode 6 communicates with the inner periphery side of the unit case 1 through the excised part 5a of the above-mentioned spacer 5, and it further communicates with the above-mentioned mesh-like spacer 14 side through a gap between the inner periphery of the unit case 1 and a perimeter edge of the diaphragm holder 4, thus being connected with the outside.
According to the microphone unit 1 shown in FIGS. 10 to 12, since the excised part 5a is formed at the spacer 5 so as to be C-shaped, there is a problem in that the spacer 5 tends to be easily transformed when assembled, leading to variations in width of the excised part 5a particularly and to difficulty in obtaining stable acoustic resistance.
Then, the applicant has proposed an arrangement of a spacer, a part of which is provided with a rebated groove, without cutting the spacer to be C-shaped as described above. This is disclosed in Japanese Utility Model Application Publication No. S61-187189. According to this, it needs a process of forming an annular groove and a sound introduction groove communicating therewith on a diaphragm holder side where the diaphragm is attached.
Further, the applicant has proposed a device in which a hole is bored by way of spark discharge at a part of the diaphragm made of a resin and pressure equalization is carried out using the hole. This is disclosed in Japanese Patent Application Publication No. H9-84195.
According to this, since a thickness of the film-like diaphragm is as thin as around 2 μm, a problem arises in that it is difficult to obtain very high acoustic resistance required for the non-directional condenser microphone in the case of attempting to apply the device disclosed in Japanese Patent Application Publication No. H9-84195 to the non-directional condenser microphone.